Minerals or debris to be extracted are generally removed from the exposed face of a deposit. The face may, for example, be the wall or the end of a tunnel, stope room, or bore. When percussive techniques are used, a tool bit is struck by a hammer mechanism, and is thereby driven against and into the face to fracture and dislodge material from the face. These techniques are widely used, and there are many tool bits for this purpose.
Existing tool bits suffer from a number of inherent disadvantages, and the bits themselves raise problems for the system in which they are used. For example, the working room available in many important mines are very limited. Tools used in such environments are often mounted so that they move along an arc, while their support vehicle crawls along a direct path. In the course of this scoop-like movement, the tool bit is driven against the face to fracture it. A percussive hammer delivers successive blows to the base of the bit. These hammers have substantial axial and lateral dimensions, and the tool bit itself adds significantly to the axial length of the assembled tool and bit.
Material on the face is most advantageously removed by a chipping action in which a blow is delivered at an acute angle to the face. There are considerable potential advantages in making this angle as large as possible, but existing tools render this difficult.
A commercially available system has been designed to overcome some of the above mentioned problems. The Voest-Alpine (VAPCUT) system applies a vibratory force to the digging teeth on buckets of shovels, backhoes, and other mining machines. The teeth associated with the buckets are widely spaced with little or no cutting taking place between them. This large space between the teeth makes the device ineffective when used in hard material such as rock. The VAPCUT system uses a special low-mass hammer that is specially designed for the system. As a consequence, other commercial impactors cannot be used because of their bulk and size.
U.S. Pat. No. 4,783,123 (Ottestad) discloses an impact ripper which uses a single impactor in combination with a single bit. This single impactor construction has the disadvantage that the cutter head size is limited by the impact energy available from the single impactor. Further, the cutter is made of steel and cannot be used in cutting hard rock.